Video: Recognizing Dangerous Lines
This video explains the different distribution lines you’ll see on utility poles—medium voltage, low voltage and telecommunications—and how dangerous they can be.
Different types of lines on utility poles
Medium-voltage lines (750–34,500 V)
Medium-voltage lines are usually located at the top of utility poles.
They are thin metal wires without an insulated sheath. They are mounted on insulators, which look like small porcelain bowls. These unsheathed lines may look harmless, but they are extremely dangerous!
Medium-voltage lines come in several possible configurations. While there are usually three separate wires, there may be up to six. There may even be a single wire, but don’t be fooled: it’s just as dangerous!
Make sure nobody and nothing—tools, equipment or building materials—ever comes within 3 m of medium-voltage lines.
Beware! At this voltage, you could suffer an electrical injury or electrocution without even touching the power lines. In fact, if you come within three metres of these power lines, a deadly electric arc could form between the lines and your body. An electric arc could also enter your body through an object you’re holding, like a ladder.
What is an electric arc? Have you ever given someone a shock by rubbing your feet on the carpet before touching them? You don’t even need to touch the other person to shock them: the static electricity circulates between the two of you, forming an arc. The same thing can happen with medium-voltage lines but with fatal results.
Why don’t medium-voltage lines have an insulating sheath?
If these are the most dangerous lines, why aren’t they sheathed?
For primarily practical and economic reasons, we rely on the air around them to provide insulation. That’s why they are strung so high, and why it’s so important not to get too close to them or let anything else get too close. If they were sheathed, they would be much bigger and heavier, and bigger or more numerous poles would be needed. The lines themselves would be more noticeable and cost much more. For these reasons, medium-voltage lines are left bare, with no insulating sheath, all over the world.
Low-voltage lines (120–600 V)
Just below the medium-voltage lines are the low-voltage lines. These are the lines that connect houses to the power grid.
The term “low-voltage lines” may make it sound like they aren’t dangerous. But they are. Even 120-V lines can transmit hundreds of amperes, enough to cause serious injury.
These are the two most common configurations of low-voltage lines:
- Two wires with a black insulating sheath, twisted around a bare metal wire
- Three stacked wires with an insulating sheath
Avoid any contact with these lines. Even though they have an insulating sheath, it may be cracked or damaged.
Less often, three bare wires, stacked one above the other, are used for low voltage. NEVER COME WITHIN 3 M OF THEM.
Below the low-voltage lines are large cables covered with a black insulating sheath. These are telecommunications lines, used for telephone and cable service.
Telecommunications lines do not belong to Hydro-Québec. They generally carry 12 volts, which is not normally an electrical hazard. However, you should not lean anything against these lines. Always avoid contact with them. In fact, a short circuit, a defect in a transformer or an electrical storm could create hazardous voltages that are strong enough to produce an electric shock that could result in injuries.
Wondering about those bigger wooden support structures or metal towers?
You’re probably looking at a high-voltage line that transmits power over long distances.
The safety rules about this type of line are even stricter.